1. Field of the Invention
The present invention relates to a method for recovering tantalum from a tantalum-containing waste, particularly to a technology for efficiently recovering tantalum from discarded substrates such as printed wiring boards containing tantalum capacitors.
2. Description of the Related Art
Tantalum capacitors have a high capacity and a high stability, and are much used, for example, for personal computers, communication devices such as mobile phones, servers and acoustic devices. Although such tantalum capacitors are mounted on various types of electronic and electric substrates such as printed wiring boards, and distributed in the market, it is the present situation that subsequent used tantalum capacitors are not actively recovered and recycled for economical and technical reasons, and are treated simply as industrial wastes.
Efforts have recently been promoted to recover rare metals from discarded electronic and electric devices and the like and reutilize the recovered rare metals due to the problem of the resource shortage. Then, tantalum being one of the rare metals is anticipated to be recovered in a high efficiency from discarded substrates such as printed wiring boards, and used tantalum capacitors.
As a technology for recovering tantalum from discarded substrates mounting tantalum capacitors, there is known, for example, a method wherein discarded substrates are subjected to a heat treatment in an oxidizing atmosphere at 550° C. or higher, and thereafter, the heat treated material is screened by the major axis length to thereby recover tantalum (Patent Literature 1). There is also known a method wherein coating materials and manganese dioxide solid electrolytes are removed from used tantalum capacitors by an acid leaching, carbon reduction-acid leaching, chlorination-distillation or argon-hydrogen plasma method, and thereafter, tantalum is refined by a chlorination, alkoxidation or electron beam melting method or a combination thereof (Patent Literature 2).
These conventional technologies can recover metallic tantalum and tantalum oxides in some quality from discarded substrates and used tantalum capacitors. However, from the tantalum recovery materials obtained by these conventional technologies, impurities such as silicon (Si), antimony (Sb), phosphorus (P), manganese (Mn), tin (Sn), lead (Pb), zinc (Zn), iron (Fe), nickel (Ni), copper (Cu) and aluminum (Al) could not sufficiently be separated; the tantalum recovery materials, in the state of containing these impurities, were difficult to reutilize as a tantalum raw material. Particularly in the case of recovering tantalum from discarded substrates, although the wastes are likely to contain a large amount of terminals, interconnects and the like containing copper, the large amount of copper (Cu) contained in these wastes was difficult to separate and remove by the conventional art methods. Also in the case where wastes contain tungsten (W) as an impurity, tungsten was difficult to sufficiently decrease by the conventional art methods.